1. Field of the Invention
The present invention relates to a heat treatment method and a heat treatment apparatus for heating a thin plate-like precision electronic substrate such as a semiconductor wafer and a glass substrate for a liquid crystal display device (hereinafter referred to simply as a “substrate”) by irradiating the substrate with a flash of light multiple times.
2. Description of the Background Art
In the process of manufacturing a semiconductor device, impurity doping is an essential step for forming a pn junction in a semiconductor wafer. At present, it is common practice to perform impurity doping by an ion implantation process and a subsequent annealing process. The ion implantation process is a technique for causing ions of impurity elements such as boron (B), arsenic (As) and phosphorus (P) to collide against the semiconductor wafer with high acceleration voltage, thereby physically implanting the impurities into the semiconductor wafer. The implanted impurities are activated by the subsequent annealing process. When annealing time in this annealing process is approximately several seconds or longer, the implanted impurities are deeply diffused by heat. This results in a junction depth much greater than a required depth, which might constitute a hindrance to good device formation.
In recent years, attention has been given to flash lamp annealing (FLA) that is an annealing technique for heating a semiconductor wafer in an extremely short time. The flash lamp annealing is a heat treatment technique in which xenon flash lamps (the term “flash lamp” as used hereinafter refers to a “xenon flash lamp”) are used to irradiate the surface of a semiconductor wafer with a flash of light, thereby raising the temperature of only the surface of the semiconductor wafer doped with impurities in an extremely short time (several milliseconds or less).
The xenon flash lamps have a spectral distribution of radiation ranging from ultraviolet to near-infrared regions. The wavelength of light emitted from the xenon flash lamps is shorter than that of light emitted from conventional halogen lamps, and approximately coincides with a fundamental absorption band of a silicon semiconductor wafer. Thus, when a semiconductor wafer is irradiated with a flash of light emitted from the xenon flash lamps, the temperature of the semiconductor wafer can be raised rapidly, with only a small amount of light transmitted through the semiconductor wafer. Also, it has turned out that flash irradiation, that is, the irradiation of a semiconductor wafer with a flash of light in an extremely short time of several milliseconds or less allows a selective temperature rise only near the surface of the semiconductor wafer. Therefore, the temperature rise in an extremely short time with the xenon flash lamps allows only the activation of impurities to be achieved without deep diffusion of the impurities.
In performing the process of activating impurities in a heat treatment apparatus including such xenon flash lamps, it is known that heating the surface of a semiconductor wafer to as high a temperature as possible by irradiating the surface with an intense flash of light achieves sufficient activation of the impurities to reduce the sheet resistance after the process. However, a device pattern is in general formed on the surface of a semiconductor wafer. There arises a problem in that the irradiation with too intense a flash of light causes the destruction of the device. For this reason, the intensity of the flash of light with which the surface of a semiconductor wafer is actually irradiated is suppressed to fall within a range where no device destruction occurs.
A semiconductor wafer with a device pattern formed thereon has a light absorptance which is pattern-dependent. Specifically, since the surface of a semiconductor wafer has a nonuniform absorptance of a flash of light, it is necessary to control the intensity of the flash of light so that the device destruction does not occur in part of the surface of the semiconductor wafer which has the highest light absorptance. However, the optimization of the intensity of the flash of light in the part having the highest light absorptance results in insufficient heating and accordingly insufficient activation of impurities in the remaining parts of the surface of the semiconductor wafer.
To solve these problems, a technique has been proposed which irradiates the surface of a semiconductor wafer doped with impurities with a flash of light multiple times (multi-flash or multi-pulse irradiation). Performing the flash irradiation multiple times allows sufficient activation of impurities over the entire wafer surface to thereby reduce the sheet resistance while suppressing the device destruction. Additionally, performing the flash irradiation multiple times allows the reduction in variations in the sheet resistance at the surface of the semiconductor wafer.
A technique which performs such a multi-flash process is disclosed in U.S. Pat. No. 6,849,831 in which pulsed light emitting lamps such as flash lamps are disposed on the front surface side of a semiconductor wafer and lamps that stay lit continuously such as halogen lamps are disposed on the back surface side thereof so that a desired heat treatment is performed using a combination of these lamps. In a heat treatment apparatus disclosed in U.S. Pat. No. 6,849,831, a semiconductor wafer is preheated to a certain degree of temperature by the halogen lamps and the like, and is then raised in temperature to a desired treatment temperature by performing pulse heating one or multiple times from the flash lamps.
Also, U.S. Patent Application Publication No. 2009/0067823 discloses an apparatus in which an insulated-gate bipolar transistor effects the on-off control of the emission of light from a flash lamp, whereby the surface of a semiconductor wafer is irradiated with a flash of light multiple times.
In the apparatus disclosed in U.S. Patent Application Publication No. 2009/0067823, the on-off control of the emission of light from a flash lamp is effected by storing electrical charges in a capacitor having a predetermined capacitance and intermittently supplying electrical charges from the capacitor to the flash lamp. However, the amounts of electrical charges which the capacitor can store are determined by capacitance and charging voltage. When the flash irradiation is performed multiple times, there are cases where sufficient amounts of electrical charges do not remain in the capacitor, in particular, during later iterations of the flash irradiation. It is impossible to recharge the capacitor during an irradiation time interval between successive iterations of the flash irradiation because this interval is a very short time interval less than one second. As a result, there arises a problem in that the intensity decreases as the number of times of the flash irradiation increases, whereby the attained surface temperature of the semiconductor wafer decreases as the number of times of the flash irradiation increases. The provision of a sufficiently high capacitance of the capacitor and a sufficiently high charging voltage allows the increase in the amounts of electrical charges which the capacitor can store, which will solve the problem. However, this results in a very large-sized power supply section including the capacitor, and significantly increases costs.